Hmmm, I have Windows 7RC on both my desktop and laptop. I've kept my desktop on and it has been running for 2 hours 40 minutes with no popups or shutdowns.
I just turned on my laptop to see if it too will stay on without popups or shutdowns, and so far I have not seen any. The last time I saw the popup was yesterday, when I had 20-something hours to go, but I haven't seen it since.
Maybe Microsoft decided not to do that? Has it actually happened to anyone yet?